1. Field of the Invention
This invention relates to an electronic circuit for use with magnetic disk storage devices, and in particular to a phase lock loop data synchronization circuit for use in an integrated circuit disk drive controller device.
2. Prior Art
For many years, magnetic disk systems have been used as data storage devices for computer systems. In particular, flexible, or "floppy", disk systems have become popular on-line storage devices for minicomputer and microcomputer systems. In the industry at present, most floppy disks are recorded using a frequency modulated (FM) data format, or a modified frequency modulated (MFM) data format. In both of these formats, data is stored and retrieved along with clocking signals as a self-clocking serial data stream on individual tracks of a floppy disk. Relatively recently, both of these data formats also have been used with certain hard disk systems and cartridge tape systems.
In both the FM and MFM data formats, each disk track is broken into sectors. Each sector is considered to be comprised of "bit cells", which define the locations on the track on which data bits may be written or read by a controller circuit. In the FM data format, each bit cell begins with a clock pulse and the contents of the center of the bit cell defines the data. If the data bit is equal to a binary zero, no pulse is written in the center of the cell. If the data is equal to a binary one, a pulse is written. For a typical eight-inch floppy disk device, each clock pulse is written four microseconds apart.
In the MFM data format, clock pulses are encoded into the data stream, and the bit cells are not physically delimited on a disk track by clock pulses. Instead, bit cells are defined relative to one another. A data bit is written into the center of a bit cell if the data is equal to a binary one, and a clock pulse is written at the leading edge of a bit cell if the preceding and succeeding data bits are both binary zero. With the MFM data format, the bit cell size is equivalent to two microseconds, but actual clock or data pulses may be two, three, or four microseconds apart. In MFM, twice as much data can be recorded without increasing the frequency rate in comparison with the FM data format.
In reading FM or MFM encoded data, both clock pulses and data pulses are read as a single serial input stream by a controller circuit. It therefore is necessary to synchronize the operation of the controller circuit to the input data pulses in order to distinguish between clock pulses and data pulses. Due to variations caused by motor speed fluctuations, read amplifier recovery circuits, and bit shifts caused by magnetic dipole repulsion of the actual magnetic domains comprising the data and clock pulses, the input pulses will tend to drift away from their nominal position on a disk track in comparison to a controller circuit Read Clock having a frequency directly related to the nominal frequency of the bit cells. Thus, a means must be used to achieve synchronization between the frequency of the Read Clock and the combined data and clock pulses being read from a disk track. One method of achieving this synchronization in the past has been a phase lock loop (PLL) circuit. However, past PLL circuits have been external to single integrated circuit disk controller devices, such as the Western Digital FD179X family of controller devices. These external PLL circuits have required a great deal of printed circuit board space, components, and engineering design effort. For example, one such external PLL circuit requires six to eight integrated circuits and numerous other active and passive electronic components.
It is thus an object of the present invention to provide an on-chip phase lock loop data synchronization circuit in conjunction with an integrated circuit disk controller device. Such an integrated circuit has the advantage of eliminating the engineering required to build an external PLL circuit, and of significantly reducing the cost of a disk controller by reducing the size of the needed printed circuit board and eliminating numerous external components.